Process for igniting and producing carbonaceous strata



United States Patent 3,034,579 PROCESS FOR IGNITING AND PRODUCING CARBONACEOUS STRATA Harry W. Parker, Bartlesvilie, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware N0 Drawing. Filed July 20, 1959, Ser. No. 828,057 11 Claims. (Cl. 166-11) This invention relates to a process for igniting subterranean gas-permeable carbonaceous strata and establishing in situ combustion therein to produce hydrocarbons therefrom.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous mate rial is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move thru the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarbon in the stratum rives out and usually upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition of carbonaceous material in a stratum around a borehole therein followed by injection of air thru the ignition borehole and recovery of product hydrocarbons and combustion gas thru another borehole in the stratum is a direct air drive process for effecting in situ combustion and recovery of hydrocarbons from the stratum. In this type of operation the stratum usually plugs in front of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difficulty and to permit the continued progress of the combustion zone thru the stratum, inverse air injection has been resorted to. By this technique, a combustion Zone is established around an ignition borehole by any suitable means and air is fed thru the stratum to the combustion zone from one or more surrounding boreholes.

In situ combustion techniques are being applied to tar sands, shale, Athabasca sand and other strata in virgin state, to coal veins by fracturing, and to strata partially depleted by primary and even secondary and tertiary recovery methods.

One of the most Widely used methods of establishing in situ combustion in a carbonaceous stratum comprises burning a fuel pack in an ignition borehole within the stratum so as to heat the adjacent stratum to combustionsupporting temperatures and contacting the hot stratum with combustion supporting gas to ignite the same. Another method utilizes a downhole heater to do the preliminary heating. In these methods the time required to heat up a sizeable section of stratum which renders ignition feasible is substantial, being as long as several days in some instances. This invention is concerned with a simple and improved method of igniting a combustible gas-permeable carbonaceous stratum.

Accordingly, it is an object-of the invention to provide an improved process for igniting a gas-permeable carbonaceous stratum. Another object is to provide an ignition process which is more rapid and more economical than methods heretofore known. A further object is to provide a process for quickly establishing in situ combustion in a carbonaceous stratum and producing hydrocarbons therefrom. It is also an object of the invention to provide a quick method of establishing inverse in situ combustion in a carbonaceous stratum. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises injecting a combustion-supporting gas containing at least 50 and preferably 95 volume percent free 0 into a gas-permeable carbonaceous stratum around an ignition borehole therein and thereafter firing an incendiary projectile into the section of stratum containing the injected gas so that the incendiary projectile heats up a portion of the stratum to ignition temperature and the combustion-supporting gas ignites the same. The combustion-supporting gas is preferably injected thru the ignition well until a substantial pressure of this gas is built up in an annular section of the stratum 10 to 20 feet or more in depth, and upon firing one or more incendiary projectiles into the annular section of stratum, the gas pressure on the ignition well is released so as to allow the gas to flow toward the ignition well and thereby expand the combustion area into which the projectile was fired. After this step, while the combustion area is still hot, additional combustion-supporting gas is injected either thru the ignition well or thru one or more offset wells so as to further expand the combustion front and establish in situ combustion, whereby hydrocarbons produced by the combustion are recovered either thru the ignition well or thru the one or more offset Wells.

It is preferred to inject combustion-supporting gas consisting essentially of free 0 (about 0 for the initial phase of the combustion step, as this concentration of free oxygen establishes vigorous combustion which effects higher temperatures and expands the combustion zone more quickly thru the stratum thereby establishing a substantial combustion area or Zone which can be expanded further by feeding air or oxygen-enriched air into the cornbustion zone either thru the ignition well or thru one or more offset Wells. When pressurizing the stratum with combustion-supporting gas, such as free 0 around the ignition borehole and releasing the pressure on the gas by venting the ignition well, the pressure may be reduced gradually or suddenly to cause the gas to flow toward the ignition Well. When venting the ignition well it is desirable to inject combustion-supporting gas thru the offset wells in applications in which inverse in situ combustion is desired. The establishment of inverse in situ combustion is greatly facilitated by incorporating in the combustion-supporting gas a suitable fuel gas in a concentration in the range of 1 to 4 volume percent. In instances in which the combustion front is rather limited in the area around the ignition well, it is almost essential to incorporate the fuel gas in the inversely injected gas in order to expand and move the combustion front thru the stratum and establish a self-propagating combustion front.

Another embodiment of the invention comprises flushing out the stratum between a ring of injection Wells and a central ignition well by injecting air thru the injection wells and producing the flushed out fluids and air thru the central well. This drives out any water in the formation and dries the same preparatory to ignition. After flushing out the intervening area, air injection is followed by the injection of slugs of oxygen or oxygen-enriched air containing at least 50 percent 0 thru each of the injection wells to drive the O -rich gas to the annulus immediately adjacent the ignition well. When the O -rich gas begins to appear in the ignition well, injection is temporarily terminated and one or more incendiary projectiles are fired into the stratum radially from the ignition Well, whereupon the injection of gas is resumed so as to expand the combustion area ignited by the heat of the incendiary projectiles in the presence of O The slugs of O -enriched gas or substantially pure oxygen may be followed by injection of air or, preferably, air containing between 1 and 4 volume percent of fuel gas, the O -rich gas effects vigorous burning and the feeding of air with or without added fuel gas expands the combustion Zone and moves the same toward the injection Wells inversely to the flow of injected gas. In this manner an inverse in situ combustion process is established in the stratum intermediate the ignition well and the injection wells, with production being recovered thru the ignition well.

In strata which are not plugged by direct drive of the combustion front with combustion-supporting gas, injection of this gas thru the ignition well is also feasible so as to drive the combustion front to the production wells in the ring. The invention is also advantageously applicable to igniting a combustible stratum for a line drive in situ combustion process. In this application the oxygen or oxygen-rich gas is injected into the stratum around alternate wells, termed ignition wells, in a line of wells in the stratum to be ignited. The other alternate wells serve as injection wells during the ignition process. The injection of the gas into the stratum adjacent the ignition wells may be effected either thru the ignition wells or thru the injection wells as described in connection with a ring type production pattern. This step may be preceded by drying out the formation with air as described heretofore. After injecting the 0 gas into the stratum around the ignition wells or between the ignition wells and the adjacent injection wells, the incendiary devices are fired into the stratum radially from the ignition wells toward each adjacent injection well and combustion-supporting gas is injected thru the injection wells and is produced thru the ignition wells so as to move the combustion fronts from the ignition wells in opposite directions toward each adjacent injection well. Continued injection of the gas moves the combustion fronts to the area around each injection well, thereby creating a combustion front line across the line of wells. Here again, the combustion fronts around each ignition well may be driven by direct drive to the adjacent wells which serve as temporary production wells.

After the combustion front line has been established across the line of wells, combustion-supporting gas is injected thru two lines of wells parallel with the line of ignition wells, with one line on each side thereof. In this manner two lines of combustion front are moved in opposite directions towards the lines of injection wells to produce the intervening stratum. If the stratum is of a nature which does not cause plugging by direct drive of the combustion fronts, this technique may be utilized in driving the fronts to the offset lines of wells which serve as production wells.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for igniting a permeable subterranean carbonaceous stratum comprising injecting free 0 into a section of said stratum; firing an incendiary projectile into said section of stratum to heat same to at least 500 F.; and flowing into the resulting hot stratum a combustionsupporting gas containing at least 50 volume percent free 0 so as to ignite said stratum.

2. The process of claim 1 wherein said gas consists essentially of free 0 3. The process of claim 1 wherein said projectile is fired radially into said stratum from an ignition well therein, said gas is pressured into said stratum thru said well before the firing, and said gas is allowed to expand into said well to contact the hot stratum.

4. The process of claim 1 wherein said projectile is fired radially into said stratum from an ignition well therein, said gas is pressured into said stratum thru said well before the firing, and said gas is forced into said ignition well by injecting air thru an offset well in said stratum.

5. A process for igniting a permeable subterranean carbonaceous stratum which comprises injecting combustion-supporting gas containing at least 50 volume percent free 0 into a section of said stratum surrounding an ignition well; firing an incendiary projectile laterally from said well into said section so as to heat a portion thereof to at least 500 F.; flowing said gas into said well while said portion is at said temperature to ignite said stratum; passing additional combustion-supporting gas into the ignited area to expand the combustion zone; and recovering produced hydrocarbons'thru said ignition well.

6. The process of claim 5 wherein said additional cornbustion-supporting gas is injected into said stratum thru an offset well radially outside of the ignited area.

7. The process of claim 5 wherein said ignition well is surrounded by a ring of injection wells, an incendiary projectile is fired from said ignition well toward each injection well, and said additional combustion-supporting gas consists essentially of air containing between 1 and 4 volume percent of fuel gas.

8. A process for igniting a permeable subterranean carbonaceous stratum along a line of alternate ignition and injection wells therein which comprises injecting combustion-supporting gas containing at least 50 volume percent 0 into a section of said stratum around each ignition well; firing an incendiary projectile into each said section from each ignition well toward each injection well so as to heat a portion of stratum in each said section to ignition temperature; passing the injected O -containing gas toward each said ignition well so as to ignite the hot stra' tum; injecting air from each said injection well thru said stratum toward each said ignition well so as to expand the combustion front and move same to the injection wells; and recovering produced hydrocarbons thru said ignition wells.

9. The process of claim 8 including the steps of injecting combustion-supporting gas thru a parallel line of injection wells on each side of said line of alternate wells after combustion fronts have been moved to each injection well in said line of alternate wells so as to feed said fronts and move same toward each line of injection wells; and recovering produced hydrocarbons thru said line of alternate wells.

10. The process of claim 8 wherein fuel gas in the range of 1 to 4 volume percent of the injected air is admixed with the injected air.

11. The process of claim 9 wherein said combustionsupporting gas consists essentially of air and fuel gas in the range of 1 to 4 volume percent of the air.

References Cited in the file of this patent UNITED STATES PATENTS 2,307,729 Foster Jan. 5, 1943 2,323,303 Bluehdorn et al. July 6, 1943 2,747,672 Simm May 29, 1956 2,880,803 Parker Apr. 7, 1959 2,899,186 Crawford Aug. 11, 1959 

